In optical networks, for example dense wavelength division multiplex (DWDM) systems, a plurality of channels are multiplexed. The term channel is used to refer to a wavelength channel, also known as an optical wavelength channel. In many such systems, a pilot tone is used for optical performance monitoring (OPM), or example to measure the power of each channel. The pilot tone is a small and low-frequency modulation (e.g. kHz to MHz) applied to a high-speed optical channel. A pilot tone provides an in-band ancillary channel for performance monitoring. Each channel is modulated with a different pilot tone (frequency). Accordingly, the power of a particular pilot tone can be useful for indicating the power of the high speed optical channel in a wavelength division multiplexed (WDM) system.
However, with conventional pilot tones, there exist a stimulated Raman scattering (SRS) effect and a chromatic dispersion (CD) fading effect. The power measurement accuracy is severely affected by these two effects. The SRS effect decreases with pilot tone frequency, whereas the CD fading effect mainly increases with pilot tone frequency. As a result, in some scenarios, it is not possible to select one pilot tone frequency with acceptable SRS and CD fading effects.
Accordingly there is a need for an improved pilot tone method and system.